1. Field of the Invention
The present invention relates to a planographic printing method and a planographic printing plate precursor used therein. Specifically, the invention relates to a planographic printing method of printing a planographic printing plate precursor by direct development in a printing machine without any development treatment step and to a planographic printing plate precursor capable of direct plate-making by scanning an infrared laser on the basis of a digital signal from a computer, etc., which is used preferably in the planographic printing method.
2. Description of the Related Art
Generally, a planographic printing plate includes a lipophilic image region receiving ink in a printing step and a hydrophilic non-image region receiving dampening water. Planographic printing is a method wherein the property of repellency between water and oil-based ink is utilized to cause a difference in adhesion of the ink to the surface of the planographic printing plate in which the lipophilic image region serves as an ink receiving part and the hydrophilic non-image region serves as a dampening water receiving part (part not receiving the ink), and the ink is allowed to adhere to only the image region and then transferred to a material to be printed such as paper.
For making such a planographic printing plate, a planographic printing plate precursor (PS plate) having a lipophilic photosensitive resin layer (image recording layer) arranged on a hydrophilic substrate has been widely used. Usually, the planographic printing plate is obtained by a method wherein the planographic printing plate precursor is exposed to light via an original image on a lithographic film or the like, and the image recording layer in the image region is allowed to remain, while the image recording layer in the non-image region is removed by dissolution with an alkali developing solution or an organic solvent, thereby exposing the surface of the hydrophilic substrate to make a printing plate.
In a plate-making process using a conventional planographic printing plate precursor, a step of removing the non-image region by dissolution with a developing solution corresponding to the image recording layer is necessary after exposure to light, and elimination or simplification of such additional wet treatment is mentioned as a task to be achieved. In recent years, disposal of waste liquid discharged in the wet treatment is a matter of high concern for the whole industry in consideration of the global environment, so there is an increasing demand for achieving this task.
In response to this demand, a method called in-machine development wherein an image recording layer from which a non-image region on a planographic printing plate precursor can be removed in an ordinary printing step is used to remove the non-image region in a printing machine after exposure to light to provide a planographic printing plate has been proposed as an easy plate-making method.
Specifically, the method of in-machine development includes, for example, a method of using a planographic printing plate precursor having an image recording layer capable of being dissolved or dispersed in dampening water, an ink solvent or an emulsion of ink and dampening water, a method which involves physical removal of an image recording layer by contact with a roller or a blanket cylinder in a printing machine, and a method which involves physical removal of an image recording layer by contact with a roller or a blanket cylinder after weakening either the cohesive force of the image recording layer or the adhesion between the image recording layer and a substrate by permeation with dampening water, an ink solvent, or the like.
Unless otherwise noted, “development treatment step” in the invention refers to a step wherein the region of a planographic printing plate precursor which has not been exposed to light from an infrared laser is removed by contact with a liquid (usually an alkaline developing solution) in an apparatus (usually an automatic developing machine) other than a printing machine, to expose the surface of a hydrophilic substrate, and “in-machine development” refers to a method and process wherein the region of a planographic printing plate precursor which has not been exposed to light from an infrared laser is removed by contact with a liquid (usually printing ink and/or dampening water) in a printing machine.
However, when an image recording layer in a conventional image recording system using ultraviolet rays or visible light is used, the image recording layer is not fixed even after light exposure, thus making it necessary to use a troublesome method wherein the exposed planographic printing plate precursor is stored in a completely shaded state or under thermostatic conditions until it is fitted into a printing machine.
In recent years, digitalization techniques which involve electronic processing, accumulation and output of image information with a computer are spreading, and a wide variety of new image output systems compatible with the digitalization techniques have come to be practically used. As a result, attention has been paid to computer-to-plate (CTP) techniques of producing a planographic printing plate directly by scanning a planographic printing plate precursor with highly directional light such as laser light carrying digitalized image information without using a lithographic film. Accordingly, it is an important technical problem to provide a planographic printing plate precursor adapted to these techniques.
As described above, simplification of a plate-making operation as well as providing a dry, treatment-free plate-making operation has been desired more strongly than in the past because of concern about both the global environment and adaptation to digitalization.
Because high-power lasers such as semiconductor lasers, YAG lasers, and the like have come to be inexpensively available in recent years, a method of using such a high-power laser as an image recording means is regarded as a promising method of producing a planographic printing plate by scanning light which can be easily adapted to digitalization techniques.
The conventional plate-making method involves imagewise exposure to light at low to medium intensity, to record an image by an imagewise change in physical properties due to a photochemical reaction in the image recording layer. On the other hand, the method of using a high-power laser involves emitting a large amount of light energy in a very short time onto a region to be exposed to light, to convert the light energy efficiently into heat energy by which the image recording layer is caused to undergo thermal change such as a chemical change, a phase change, or a change in form or structure, and then utilizing the change in image recording. Accordingly, although the image information is outputted by light energy such as laser light, image recording is conducted not only by light energy but also by heat energy. Usually, the recording system using generation of heat by exposure to high-power density light is called heat mode recording, and conversion of light energy into heat energy is called light/heat conversion.
A great advantage of the plate-making method using heat mode recording is that the image recording layer is not sensitive to light at an ordinary intensity level such as interior illumination, and also that fixation of an image recorded by exposure to high-intensity light is not essential. That is, the planographic printing plate precursor used in heat mode recording is not sensitive to indoor light before light exposure is carried out, and fixation of the resulting image after light exposure is carried out is not essential. Accordingly, the plate-making process wherein an image recording layer to be made insoluble or soluble by exposure to light from a high-power laser is exposed to imagewise light to form a planographic printing plate can be carried out using in-machine development, thereby realizing a printing system wherein the image is not influenced even by exposure to indoor ambient light. Accordingly, it is expected that a planographic printing plate precursor used preferably in in-machine development can be obtained by utilizing heat mode recording.
The development of lasers in recent years has been remarkable, and in particular, high-power, small-size solid lasers and semiconductor lasers emitting infrared rays of wavelengths of 760 to 1200 nm can be easily obtained. These infrared lasers are very useful as recording light sources for direct plate-making by digital data from computers, etc.
However, many photosensitive recording materials that are practically useful as the image recording layer have photosensitive wavelengths in the visible light range of 760 nm or less, and therefore cannot be used in recording an image with an infrared laser. Accordingly, there is a need for materials capable of image recording with an infrared laser.
As such an image recording material, a planographic printing plate precursor having a hydrophilic substrate on which an image recording layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder is arranged has been proposed in Japanese Patent No. 2938397. This planographic printing plate precursor can be exposed to light from an infrared laser to thermally fuse the hydrophobic thermoplastic polymer particles to form an image, fitted onto a cylinder in a printing machine, and subjected to in-machine development with dampening water and/or ink, and exhibits good in-machine development properties. However, in the method of forming an image in this manner by mere thermal fusion of fine particles, there is a problem in that the image intensity, and particularly the adhesiveness of the substrate to the ink receiving layer, is very low, and printing durability is insufficient.
As other examples of such planographic printing plate precursors that can be subjected to in-machine development, a planographic printing plate precursor containing microcapsules incorporating a polymerizable compound on a hydrophilic substrate (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2001-277740 and 2001-277742) and a planographic printing plate precursor provided with a photosensitive layer containing an infrared absorber, a radical polymerization initiator and a polymerizable compound on a substrate (see, for example, JP-A No. 2002-287334) have been proposed.
An image region produced by the method of forming an image in this manner by polymerization reaction has a higher density of chemical bonds in the image region than in an image region formed by thermal fusion of fine polymer particles and is thus inherently relatively excellent in image density. However, from a practical standpoint, the method of using a polymerization reaction is still insufficient in in-machine development properties, printing durability and polymerization efficiency (sensitivity) and is not practically usable.